Sack is derived from the term "sack of shit". It describes someone who is unworthy of even being filled the feces that would reside within a "sack of shit". It is usually used to describe a person who is extremely lazy or is performing an extremely lazy act.
Friend #1: "Hey man can you shut the lights off?"

Friend #2: "Sorry bro. Busy."

Friend #1: "Dude you're right next to them. Just reach over and shut them off."

Friend #2: "Man that's too far away. I'm busy."

Friend #1: "Busy doing what? You've been playing League of Legends and eating toaster waffles on my couch for six hours."

Friend #2: "Exactly. I'm busy."

Friend #1: "You're a fucking sack."
by Kflop Fats January 25, 2013
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To like something.
To diggg it.
mainly used in northern virginia.
1) girl i sack your new top, it's so cute!
2) man that chick is sackin' me hardcore, she calls me everynight!
3) i know you sack her back!
by elz bellz November 4, 2007
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Verb, mainly Scottish. The act of hitting a man in the testicles.
to sack

''Shut it you slag, or I will sack you.''
by diegothehumanrocket April 23, 2006
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Sack can be used in a number of ways:
1) to insult someone
2) to refer to balls
3) to describe a beating / misfortune
4) to subsitute the word 'shit'
1) dude, that guy is such a sack
2) lick my sack
3) that guy just got completely sacked by that crazy mofo
4) ohh sack that hurt
by ryan tanzi May 10, 2008
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this verb means to dump someone's bookbag out clean.
guy 1 "Hey, Jim left! sack him."

guy 2 (so guy 2 dumps everything out of Jim's bag)
by funnyman21224 July 31, 2010
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